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Research update: Cochlear Implants

by: Dr. Bob Carlyon, Deputy Director
MRC Cognition & Brain Sciences Unit
15 Chaucer Rd.
Cambridge CB2 7EF  England
 

 

 

 

 

 

 

 

As it stands

 

Cochlear implants (CIs) have restored hearing to more than half a million deaf people worldwide. A typical CI processor extracts the slowly varying envelope in each frequency band; each envelope then amplitude-modulates a train of electrical pulses applied to one of an array of electrodes inserted in the cochlea, with higher-frequency bands applied to more basal electrodes. Variations on this theme include so-called n-of-m strategies, where in each short time window only a subset of electrodes, corresponding to the more energetic bands, are activated, and fairly recent strategies that convey information on the temporal fine structure of the lower-frequency channels.

 

Many CI patients understand speech well in quiet, but performance varies markedly across listeners, with a significant minority struggling to understand speech even in the most favourable conditions. Even for the most successful patients, speech perception deteriorates markedly in noisy situations. Pitch perception is poor, and this contributes to the speech-in-noise problem, greatly reduces the enjoyment of music, and degrades the understanding of tonal languages. An overview of CIs can be found here.

 

Coming soon

 

In everyday clinical use, all contemporary CIs stimulate each intra-cochlear electrode in so-called monopolar mode, whereby current is returned by one or more electrodes outside the cochlea. This causes a broad current spread leading to a loss of spatial selectivity; each electrode stimulates a wide range of auditory nerve fibres, including those near distant electrodes. In principle the current spread can be reduced using more-focussed modes of stimulation, whereby current is returned via one or more intra-cochlear electrodes. Despite promising results from animal studies, research with human CI listeners has revealed only modest and variable benefits to speech perception. Psychophysical studies also reveal only modest improvements to spatial selectivity, and these can differ across electrodes, even for the same listener. Indeed, psychophysical and electrophysiological studies have revealed substantial across-electrode variations not only in spatial selectivity but also in detection thresholds, gap detection, modulation detection, and pitch perception. This has led to the suggestion that, rather than adopt a one-size-fits-all approach, CIs could be programmed in a bespoke manner so as to improve performance for each patient. For example, subsets of electrodes could be either de-activated or stimulated using a focussed mode, depending on the results of psychophysical and/or electrophysiological tests. Finally, an increasing number of patients are being implanted with residual hearing in either the implanted or non-implanted ear. There has been a steady increase in our understanding of how best to combine this acoustic hearing with that provided by the CI

 

Maybe one day

 

We know from psychophysical experiments that there are substantial biological limitations that limit hearing by CI users, even with idealised stimuli. The limited success of attempts to improve spatial selectivity using focussed stimulation is likely to be at least partly due to incomplete survival of the auditory nerve. Temporal processing is also limited: even with the simplest stimulus – a periodic pulse train applied directly to a single CI electrode – CI patients are typically unable to discriminate changes in pulse rate above about 300 pulses per second (pps). There is some evidence that this limit arises from neural processes in the auditory brainstem. Discrimination even of more modest rates, e.g. close to 100 pps, is substantially worse than frequency discrimination of pure tones in normal hearing. These limitations in temporal processing are likely to scupper attempts to code temporal fine structure in new CI processing strategies. Both the spatial and temporal limitations make it unlikely that substantial improvements can be obtained simply by tweaking existing technology, methods of stimulation, or processing strategies.

 

Radical new methods for stimulating the nerve, so far tested only in animal experiments, include optogenetics and the use of an electrode array that penetrates the nerve bundle itself, rather than residing in the cochlea. This latter approach produces greater spatial selectivity than intra-cochlear stimulation and also requires less current. One caveat is that, in patients with substantial auditory nerve loss, selective stimulation may not produce a sufficiently loud percept. This would impose a limit on the spatial selectivity obtainable in these patients, no matter what the technology.

 

Finally, watch out for new pharmaceutical interventions that may improve the status of the auditory pathway. One new approach involves drug delivery to the cochlea in association with a CI.  This research concentrates on the application of drugs to reduce the foreign body reaction, to rescue auditory neurons and to promote the sprouting of their neurites towards the electrode array, and to preserve residual hearing following implantation in patients with some level of useful hearing. There is also an ongoing trial of a fast-acting potassium-channel modulator – which may enhance fast temporally accurate firing throughout the auditory system – in CI patients. These interventions, if successful, may unlock the potential of manipulations – such as focussed stimulation and new processing strategies– that are currently of limited use with the degraded auditory systems of many CI patients.

 

Thanks to Rob Shepherd for advice. Conflict of interest. The author’s lab is involved in a drug trial funded by Autifony Therapeutics. The author recently acted as consultant for Advanced Bionics on a project on combined acoustic and electric (CI) hearing. 

 

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